Liquid-liquid extraction techniques provide for the extraction of a liquid from a solution by intimate contact of that solution typically in a countercurrent manner and typically in an extraction column containing means to promote intimate contact of the solution with another solution in which one or more components of the first solution are preferentially soluble, and which second solution is immiscible with the first solution or at least only partially of limited solubility therein. By repeated and intimate contact of the first and second solutions for example, in a column containing sieve trays, wherein one solution may be the heavy phase, such as a solution of methyl ethyl ketone and water, and the second solution is the light phase, such as toluene, whereby, by intimate mixing of the heavy phase as it passes downwardly in the column with the light phase as it passes upwardly in the column, extraction of the methyl ethyl ketone by the toluene is accomplished with a methyl ethyl ketone-toluene solution removed from the top of the column and the material or by-product water removed from the bottom of the column. The liquid-liquid extraction apparatus and techniques are well known and have been used in the past (see, for example, "Selection Criteria for Liquid-Liquid Extractors," Chemical Engineering, Nov. 6, 1978, pp. 109-117). However, the practical efficiency of liquid-liquid extraction apparatus and methods and difficulties associated with scale-up problems has limited the widescale adoption of liquid-liquid extraction as a chemical-separation technique.
In liquid-liquid extraction methods, the heavy phase is referred to as that phase which flows downwardly in the extraction column; that is, the phase of greater density, and the light phase is that phase which flows upwardly in the extraction column; that is, the phase of less density. In liquid-liquid extraction columns, the columns typically contain a plurality of trays and/or packing material, such as, for example, a series of columns containing packing material or a column containing a plurality of sieve trays, which provide for the intimate mixing of the heavy and light phase on the surface of each tray.
Numerous techniques have been tried in order to increase the efficiency of liquid-liquid extraction techniques. For example, pulsing of the phases in the column, to provide the intimate mixing of liquid and vapor on trays, has been suggested to improve the efficiency of separation processes (see, for example, the Oil and Gas Journal, July 28, 1962, pp. 268-271, 318-319, "Distillation, Absorption, and Extraction Column," M. R. Cannon). In addition, controlled-time cycling has been suggested for extraction columns in order to improve the capacity and efficiency of a column. In such suggested techniques, both the heavy and light phases feeding into the column were fed continuously and then were pulsed through the pulsing of the entire volume of the column. This technique required the employment of very large pumps. In addition, it causes a drop in the interface between the light and heavy phases past some of the trays employed in the column; that is, as the column is pulsed, the interface of the system drops, due to what is referred to as a "gap," but which in essence is caused by phase-volume depletion; that is, phase volume decrease or increase of the respective phases (see The Canadian Journal of Chemical Engineering, Vol. 19, February 1971, pp. 95-104, "Controlled Cycling Extraction," C. R. Darsi and J. E. Feick).
Thus, in the past, previous attempts, to utilize liquid-liquid extractors and techniques effectively, have been cumbersome, complicated and expensive and not wholly successful. One particular problem associated with liquid-liquid extraction techniques arises because, by definition, material is extracted from one phase to another phase; that is, from the light to the heavy or from the heavy to the light phase. This extraction causes phase-change-volume depletion and phase-volume decrease or buildup in the respective phases. As in the controlled-cycling extraction technique of Darsi et al. (supra), if no correction of this change in phase volume occurs, then the phase interface drops or moves upwardly, leading to reduced efficiency of the column.
Attempts have been made to overcome phase-volume depletion and buildup by interrupting the liquid-liquid extraction operation periodically and adding or recycling the depleted phase through the extractor, while draining out and removing the buildup or increase in phase. This technique is detrimental to continuous operation and is quite cumbersome. Attempts have been made to solve the problem by employing a third fluid, such as the use of a gas, such as inert nitrogen, to make the necessary corrections in phase volume. Therefore, there exists a need to improve liquid-liquid extraction methods and apparatus, which methods are simple, easy and effective, during operation of the liquid-liquid extraction, to control the change in phase volume occurring with such liquid-liquid extraction.